Any intervention in Syria would not be about the conflict itself but preventing the use of chemical weapons by any regime, David Cameron said today, stressing no decisions about British involvement have been made.

The Prime Minister said there was never 100% certainty or a single piece of irrefutable evidence but said the world had agreed almost a century ago chemical weapons should not be used.

Decisions about British involvement have not been taken, he said, adding Parliament was the "right place to set out all of the arguments". He said action must be "proportionate, have to be legal, would have to specifically be about deterring the use of chemical weapons".

Mr Cameron said any action had to be legal, proportionate and a deterrent to the future use of such weapons. He added: "Let me stress to people, this is not about getting involved in a Middle Eastern war or changing our stance in Syria, or going further into that conflict. It's about chemical weapons. Their use is wrong and the world should not stand idly by."

Mr Cameron said the question for Britain is whether failing to act this time would lead to more use of chemical weapons in Syria and elsewhere in future. "It must be right to have some rules in our world and try to enforce those rules," he said. "Of course as Prime Minister I take my responsibilities about the safety of our Armed Services incredibly carefully, seriously but the question we need to ask is whether acting or not acting will make the use of chemical weapons more prevalent."

Mr Cameron said Thursday's debate would ensure "proper" scrutiny and allow the Government to listen to MPs. He said: "Obviously this is a developing situation, as I say, decisions have not been taken, but we shouldn't stand by when we see this massive use of chemical weapons and appalling levels of suffering. I think in Parliament is the right place to set out all of the arguments, all of the questions. But I would say this to people - there is never 100% certainty, there is never one piece or several pieces of intelligence that give you absolute certainty.

"But what we know is this regime has huge stocks of chemical weapons. We know they have used them on at least 10 occasions prior to this last widescale use. We know they have both the motive and the opportunity whereas the opposition does not have those things and the opposition's chance of having used chemical weapons in our view is vanishingly small."

Labour leader Ed Miliband said: "When I saw the Prime Minister this afternoon, I said to him that we the Labour Party would consider supporting international action, but only on the basis that it was legal, that it was specifically limited to deterring the future use of chemical weapons, and that any actions contemplated had clear and achievable goals. And we'll be scrutinising any action that is contemplated on that basis."

Separately, the British public is against the UK becoming involved in the Syria crisis, a survey has found. A YouGov survey for The Sun revealed that nearly three-quarters (74%) oppose deploying British troops to the conflict-torn country. The poll found that half (50%) of Britons oppose attacking with long range missiles from ships, while just a quarter (25%) are in favour of it.

It showed that British military involvement is unpopular no matter what political party people support, with Ukip voters the most strongly against it at 68%. And it found that a majority of almost 3-1 also think the Government should be bound by Parliament's Thursday vote on whether Britain should go to war.